Scanning electron micrograph of a cross section of heartwood of the oak, Quercus robur. The picture shows the vascular tissue, bounded by medullary rays (dark vertical edges). Rays run spoke-like from the centre of the trunk - beyond the bottom of this picture - to its outer edge. The vascular tissue shows parenchyma (small cells) together with xylem vessels, that conduct water. The largest vessel here (right, below centre) is 0.3mm across. Six vessels show ingrowths (tyloses), that develop when adjacent cells grow through pits in the vessel wall. Tyloses block movement of water and pathogens, and confer durability to the timber. The large vessels developed in Spring; the smaller ones (above), in Autumn. The difference in size is visible to the naked eye; the picture thus represents one "annual ring". Annual rings vary in width, and record the climatic conditions as they were formed.